Small, enigmatic alligatoroid from the Middle Eocene Clarno Formation, John Day Fossil Beds, Oregon

Detrital laterites interbedded with clayey, Ultisol-like paleosols in the late Eocene strata of central Oregon record periods of soil erosion, colluvial concentration of iron-cemented soil nodules, and deposition of these weathered products in hillslope settings. Two sets of lateritic paleosols are extensively exposed in the Painted Hills area of Oregon and span the transition from Eocene Clarno Formation andesitic volcanism to the initiation of late Eocene–Miocene pyroclastic volcanism of the John Day Formation. These late Eocene lateritic paleosols developed along the margins of several different lava flows where they formed local accumulations of iron-rich strata, which are now exposed in deep-red and ocher-colored badlands along the exhumed flow margins. Stratigraphically, the lateritic paleosols are above upper Clarno Formation rhyodacite flows, below the thick tuffaceous Oligocene–early Miocene part of the John Day Formation, and sandwich the welded tuff of member A that defines the base of the John Day Formation. In each of several cases from different lava flows studied, a similar sequence of detrital laterites and clayey paleosols rest on weathered lava flow breccia. The basal paleosol of these sequences consists of a thick (5–10 m), very strongly weathered saprolite zone developed in lava flow breccia and an overlying clayey B horizon. This paleosol is overlain by 8–12 m of alternating clayey, kaolinite-rich paleosols (Ultisol-like paleosols) and weakly developed paleosols with iron-rich, claystone breccia fragments (detrital laterites). The iron-cemented claystone fragments are up to 35% Fe 2 O 3 , very base poor, and weather-resistant, and they contain abundant cross-cutting clay skins and clay-filled pedotubules indicative of polycyclic weathering. The lower of the two sets of detrital laterites is associated with a thick rhyodacite flow in the upper Clarno Formation and has an up-section increase in the degree of weathering and concentration of resistate constituents, as determined by mass-balance geochemical analysis. The time span represented by this well-developed weathering trend is estimated to be between 2 and 4 m.y. based on estimates of the time of formation of interbedded paleosols. This long-lasting weathering trend is probably the result of a lack of soil rejuvenation resulting from the late Eocene hiatus between Clarno and John Day volcanism. A developmental model for the formation of the detrital laterites and Ultisol-like paleosols involves alternating episodes of soil formation and soil erosion in which iron-rich soil nodules are concentrated as a colluvial lag deposit on the toe slope of hills. Subsequent colluvial pulses of iron-cemented gravel were increasingly weathered and rich in resistate constituents because of longer residence time in up-slope soils. During periods of landscape stability, slow vertical accretion of soils by small additions of volcanic ash and dust produced the strongly developed, but nonlateritic, Ultisol-like paleosols. The episodes of soil erosion probably correspond to periods of climatic change during the late Eocene climatic deterioration. The John Day Formation detrital laterites and clayey paleosols are very similar to the Clarno formation laterites except for the presence throughout the section of 1%–3% pyrogenic feldspar crystals. No up-section increase in weathering is observed in the John Day detrital laterites, perhaps because of rejuvenation of soils by volcanic ash. The similar textures and chemistries of the two groups of detrital laterites, despite the onset of John Day pyroclastic volcanism, indicate that climate remained subtropical and humid up to the Oligocene-Eocene boundary.

The John Day Formation of central and eastern Oregon, USA, contains a rich record of late Eocene to early Miocene faunal and floral evolution, climate and environmental change, and landscape evolution, and accordingly, it has been studied for over a century to better understand ancient terrestrial ecosystems of North America. Further progress in leveraging the John Day Formation rock archive to study these systems requires an updated chronostratigraphic framework with a foundation of modern high-precision radioisotope geochronology. We present a comprehensive dataset of U-Pb zircon dates measured by chemical abrasion−isotope dilution−thermal ionization mass spectrometry that yielded 23 new highly precise eruption and depositional ages for volcanic beds in the John Day Formation stratigraphy, in the environs of John Day Fossil Beds National Monument. The new ages significantly refine the overall timing of the John Day Formation and reveal new spatiotemporal correlations among strata of the Sheep Rock, Painted Hills, and Clarno Units. We integrated our dataset with composite stratigraphic information through Bayesian age modeling to establish probabilistic posterior age constraints for all formal and informal lithostratigraphic divisions of the eastern facies of the John Day Formation and durations of previously identified long-lived hiatuses. The results robustly link the John Day Formation stratigraphy to the geologic and paleomagnetic time scales and global proxy records, offering new opportunities for investigating terrestrial records of the Eocene−Miocene Earth system in the Pacific Northwest and testing diachroneity of faunal assemblages that define North American Land Mammal Ages.

One of the iconic fossils of the John Day Fossil Beds National Monument, Oregon, USA, is the Hancock Tree—a permineralized standing tree stump about 0.5 m in diameter and 2.5 m in height, embedded in a lahar of the Clarno Formation of middle Eocene age. We examined the wood anatomy of this stump, together with other permineralized woods and leaf impressions from the same stratigraphic level, to gain an understanding of the vegetation intercepted by the lahar. Wood of the Hancock Tree is characterized by narrow and numerous vessels, exclusively scalariform perforation plates, exclusively uniseriate rays, and diffuse axial parenchyma. These features and the type of vessel-ray parenchyma indicate affinities with the Hamamelidaceae, with closest similarity to the Exbucklandoideae, which is today native to Southeast and East Asia. The Hancock Tree is but one of at least 48 trees entombed in the same mudflow; 14 others have anatomy similar to the Hancock Tree; 20 have anatomy similar to Platanoxylon haydenii (Platanaceae), two resemble Scottoxylon eocenicum (probably in order Urticales). The latter two wood types occur in the nearby Clarno Nut Beds. Two others are distinct types of dicots, one with features seen in the Juglandaceae, the other of unknown affinities, and the rest are very poorly preserved and of unknown affinity. Leaf impressions in and immediately below the layer containing the trees include the extinct genera Macginitiea and Platimeliphyllum (Platanaceae), and Trochodendroides (Saxifragales).

Wiregrass (Ventenata dubia [Leers] Coss.), an annual grass from the Mediterranean region of North Africa and Eurasia that has aggressive invasion potential in many North American plant communities, has only recently been reported in low-elevation sagebrush steppe. We first encountered wiregrass in 2014 in the John Day Fossil Beds National Monument, a low-elevation steppe protected area in central Oregon. This discovery was incidental to formal vegetation monitoring that was initiated in the monument in 2009. We first encountered wiregrass in monitoring plots in 2016, and, from plot data, we documented rapid spread during 2017–2019. Wiregrass infestation increased within our 4674-ha monitored area from 21 ha (95% CI, 3 to 106 ha) in 2016 to 138 ha (95% CI, 31 to 265 ha) in 2018, and declined to 63 ha (95% CI, 13 to 119 ha) in 2019, representing a cumulative increase of 300% over the 4-year period. Variation in weather may explain this annual variation in wiregrass. We examined mean monthly water balance deficit during the autumn, winter, and spring preceding each survey year and found evidence of a potential correlation between winter deficit and wiregrass. The lowest winter deficit occurred in 2018 prior to the survey documenting the largest wiregrass increase. Wiregrass exhibited a broad ecological niche within our survey area, occurring across all surveyed elevations and on all but steep southern slopes. Invaded sites were in well-drained clay soils in association with other invasive annual grasses. Our observations contribute to the growing evidence that wiregrass poses a greater threat to low-elevation sagebrush ecosystems than previously recognized. It also illustrates the kinds of external stressors that are impacting sagebrush steppe protected areas and the need for continued early detection and rapid response measures, as well as long-term monitoring of invasions and response effectiveness.

Differential Species Responses to Aspects of Resistance to Invasion in Two Columbia Plateau − Protected Areas

The rock formations of the John Day Fossil Beds National Monument area are aquifers that can be expected to yield less than 10 gallons of water per minute to wells. The most permeable of the geologic units is the alluvium that occurs at low elevations along the John Day River and most of the smaller streams. Wells in the alluvial deposits can be expected to yield adequate water supplies for recreational areas; also, wells completed in the underlying bedrock at depths ranging from 50 to 200 feet could yield as much as 10 gallons per minute. Pumping tests on two unused wells indicated yields of 8 gallons per minute and 2 gallons per minute. Nine of the ten springs measured in and near the monument area in late August of 1978 were flowing 0.2 to 30 gallons per minute 0 Only the Cant Ranch spring and the Johnny Kirk Spring near the Sheep Rock unit had flows exceeding 6 gallons per minute. Chemical analyses of selected constituents of the ground water indicated generally low concentrations of dissolved minerals. Although cloudbursts in the Painted Hills unit could generate a flood wave on the valley floors, flood danger can be reduced by locating recreational sites on high ground. The campground in Indian Canyon of the Clarno unit is vulnerable to cloudburst flooding. About 80 percent of the proposed campground on the John Day River in the Sheep Rock unit is above the estimated level of 1-percent chance flood (100-year flood) of the river. The 1-percent chance flood would extend about 120 feet from the riverbank into the upstream end of the campground 0

Successions of paleosols bounded by erosional surfaces in fluvial sediments of the Eocene‐Oligocene strata of Central Oregon can be interpreted as terrestrial equivalents of the unconformity‐bound units of sequence stratigraphy. In the upper part of the upper Eocene Clarno Formation and in the lower part of the lower Eocene‐lower Miocene John Day Formation, truncation surfaces separate otherwise conformable alluvial deposits and allow for stratigraphic subdivision into informal members (lower and upper “Red Hill” claystones in the Clarno Formation and lower, middle, and upper Big Basin Members and lower Turtle Cove Member in the John Day Formation). Paleosols in each member show a stepwise change in the degree of weathering of the most strongly developed paleosols: kaolinite‐rich, Ultisollike paleosols in lower “Red Hill” claystones (late Eocene, 42‐43 Ma), smectite‐rich Alfisol‐like paleosols in the upper “Red Hill” claystones (late Eocene, 41‐42 Ma), Alfisols and Ultisol‐like paleosols in the lower Big Basin Member (late Eocene, 34–40 Ma), Alfisol and Inceptisol‐like paleosols in the middle and upper Big Basin Members (early Oligocene, 30–34 Ma), and calcic Inceptisol‐like paleosols in the lower Turtle Cove Member (middle Oligocene, 28–30 Ma). These changes across the Eocene‐Oligocene transition are interpreted as representing global cooling and drying of the midlatitudes from Eocene subtropical, humid conditions to Oligocene temperate, subhumid conditions. In central Oregon, these changes appear to be stepwise with climatically stable periods, represented by packages of similar paleosols, of approximately 2‐4 m.y. in duration. Our interpretation of these paleosol packages as non‐marine sequences is not based on correlation with sea‐level changes but on correlation with global climate change events. Geomorphic processes influenced by climate and vegetation, and not base‐level change, basin subsidence, or volcanic supply are thought to have controlled sedimentation rates. Thus, the stepwise increase in sedimentation rates across the Eocene‐Oligocene transition in the central Oregon alluvial strata reflect increased sediment yields due to drying climatic conditions. High‐precision 40Ar/39Ar age determinations of tuffs allow for the correlation of these sequences with the record of global climate change from deep sea cores. Three major paleoclimatic changes stand out. The change from Ultisol‐like paleosols formed in near‐tropical climate to AAlfisol‐like paleosols formed in subtropical climate between 42.8 and 43 Ma) corresponds to a global cooling trend after the mid‐Eocene climatic optimum. The Eocene‐Oligocene boundary (∼34 Ma) is marked by the change from subtropical Ultisol‐like paleosols to Alfisol‐like paleosols formed in temperate humid climate. Global cooling during the mid‐Oligocene (∼30 Ma) is reflected in a change from non‐calcareous, Alfisol‐like paleosols to calcareous Andisol‐like paleosols formed in sub‐humid temperate conditions. These mid‐Tertiary paleosol sequences are evidence of stepwise terrestrial climate change that was strongly coupled with marine events.

We describe the roosting and foraging behavior patterns of western small-footed myotis (Myotis ciliolabrum) observed during a vertebrate inventory of the John Day Fossil Beds National Monument in north central Oregon. We used radiotelemetry to track 9 adult females, including 3 lactating and 6 postlactating bats, during July-September 2003. We found that these bats showed considerable fidelity to a common foraging area at the confluence of the John Day River and a tributary creek along which bats commuted and roosted. Individual bats did not roost together, but each showed high fidelity to local clusters of rock outcrops in small side canyons along the tributary. Roosts were not found in large, exposed cliff faces, despite the availability of such features. Rather, radio-tagged bats roosted in smaller outcrops that averaged 4.5 m in height. Bats commuted up to 12 km from roosts in the tributary canyon to the common foraging site at the river confluence and remained on the wing to forage for up to 4 h before returning to day roosts. No radio-tagged bats were observed using night roosts, even after pups were weaned. Our study provides a description of roosting, commuting, and foraging activity, as well as habitat use, of western small-footed myotis. This information provides a nuanced perspective on the ecology of canyon-dwelling bats in the region. Such perspective could be useful for conservation and habitat management.

We diagnose a new species of Brontotheriidae from a middle Eocene locality, the Clarno Nut Beds, from the Clarno Formation, John Day Basin, Central Oregon. Though renowned for its richness in fossil flora, fossil vertebrates are rare in the Clarno Nut Beds and this new species is the most abundantly represented mammal. Radiometric dating constrains the age of the Nut Beds fauna to about 43.76 Ma within the Uintan North American Land Mammal Age. This new taxon, represented by numerous cranial, mandibular, and dental specimens, is comparatively small for a brontothere and notable for its cranio-caudally shorted nasal incision, a trait shared with three larger-bodied middle Eocene species,Metatelmatherium ultimum,Wickia brevirhinus, andSthenodectes incisivum. Phylogenetic analysis suggests the sister taxon of the Nut Beds brontothere could be one of two species—Wickia brevirhinusfrom the Sand Wash Basin of Colorado and Washakie Formation of Wyoming, orMetatelmatherium ultimum, a pan-Beringian species known from the Uintan Formation of Utah (and other Uintan age deposits) and the “Irdin Manha” Formation of Inner Mongolia, China. Phylogenetic results also indicate that the Nut Beds brontothere is a dwarf taxon. Though brontotheres are renowned for having evolved very large body sizes, this new brontothere is one of several discovered in recent decades that suggest evolutionary reductions in body size may have been relatively common in Brontotheriidae.

The Standard of California 1 Kirkpatrick well, near Condon, Oregon, penetrated 2440 ft of Columbia River Basalt (CRB), 4255 ft of John Day Formation-Clarno Formation(.), and was abandoned after penetrating 2031 ft of Mesozoic sedimentary rocks. The CRB section is entirely Grande Ronde Basalt, and probably contains all four magnetostratigraphic units. Prineville and Picture Gorge basalts, which crop out less than 15 mi away, are absent. Below the CRB are the volcanic rocks of the John Day Formation. Below the ash-flow tuff member A are 560 ft of either John Day or Clarno Formation which, on the basis of petrography, isotopic age dates, and flow compositions, they interpret as a previously unrecognized part of the John Day Formation. Part of a 28 m.y.-old rhyolitic intrusion is interpreted to occur in the Mesozoic rocks and lower John Day Formation-Clarno Formation(.) section. The Tertiary stratigraphy at the well appears to be controlled by uplift of the Blue Mountains and by local deformation. The John Day Member A occurs at about 2000 ft elevation near Fossil, Oregon, and in the well at about 3300 ft below MSL; this suggests that over 5300 ft of uplift occurred over the last 37 m.y. in themore » Blue Mountains, relative to the Kirkpatrick area. The area coincides with a steep gravity gradient along the north flank of the Blue Mountains (Riddihough, 1984), which suggests a fault. Uplift began during the Clarno as indicated by paleocurrent directions in pre-Clarno rocks near Heppner, Oregon, and continued beyond the Miocene, controlling the CRB and producing the north-dipping homocline.« less

Scenic red color-banded claystones of the Clarno and Painted Hills areas of central Oregon are successions of fossil soils that preserve a record of Eocene-Oligocene paleoclimatic change. Conglomerates of the middle Eocene Clarno Formation near Clarno contain largely weakly developed paleosols compatible with an environment of volcanic lahars around a large stratovolcano. Deeply weathered paleosols (Ultisols) around a volcanic dome and overlying these conglomerates indicate a climate that was subtropical (mean annual temperature or MAT 23-25° C) and humid (mean annual precipitation or MAP of 900-2,000 mm). Comparable paleoclimates are indicated by fossil floras from the conglomerates, which show diversity and adaptive features similar to modern vegetation of Volcan San Martin, Mexico. An erosional disconformity in the Clarno area separates these older beds from less deeply weathered red paleosols (Alfisols) in the middle Eocene upper Clarno Formation. The change in paleosols may represent a decline in both temperature (MAT 19-23° C) and rainfall (MAP 900-1,350 mm), with dry seasons. Strongly developed lateritic paleosols (Oxisols and Ultisols) in the uppermost Clarno and lowermost John Day Formations in the Painted Hills record return to more humid conditions during the late Eocene. These paleosols are similar to soils of southern Mexico and Central America in climates that are subtropical (MAT 23-25° C) and humid (MAP 900-2,000 mm). Kaolinitic and iron-rich, red paleosols (Ultisols) of the lower Big Basin Member of the John Day Formation near Clarno and the Painted Hills are erosionally truncated and abruptly overlain by smectitic and tuffaceous paleosols (Inceptisols and Alfisols) of the middle Big Basin Member. This truncation surface can be correlated with the local Eocene-Oligocene boundary. Paleosols of the middle Big Basin Member are most like those of the Central Transmexican Volcanic Belt and indicate an early Oligocene paleoclimate appreciably cooler (MAT 16-18° C) and drier (MAP 600-1,200 mm) than during the late Eocene. Root traces and clay accumulations in the paleosols indicate forest vegetation, also evident from fossil leaves of the lake-margin Bridge Creek flora. The mid-Oligocene upper Big Basin Member of the John Day Formation includes distinctive brown as well as red paleosols (Alfisols). Its paleosols indicate a paleoclimate drier (MAP 500-700 mm) than before, and more grasses in the forest understory. Another erosional truncation marks the base of the late Oligocene (early Arikareean), olive-brown lower Turtle Cove Member of the John Day Formation. Calcareous paleosols with near-granular soil structure (Inceptisols and Aridisols) indicate an even drier (MAP 400-600 mm) climate, more open grassy woodland vegetation than previously, and local wooded grassland of seasonally wet bottomlands. The Clarno-John Day sequence preserves a long-term paleoclimatic record that complements the geological record of global change from deep sea cores and fossil plants. Similarly, it reveals stepwise climatic cooling and drying, with a particularly dramatic climatic deterioration at the Eocene-Oligocene boundary.

Painted Hills, John Day Fossil Beds National Monument, Oregon

Following their footsteps: Report of vertebrate fossil tracks from John Day Fossil Beds National Monument, Oregon, USA

Contrasting Effects of Long-Term Fire on Sagebrush Steppe Shrubs Mediated by Topography and Plant Community

The structure and composition of sagebrush‐dominated ecosystems have been altered by changes in fire regimes, land use, invasive species, and climate change. This often decreases resilience to disturbance and degrades critical habitat for species of conservation concern. Basin big sagebrush (Artemisia tridentata ssp. tridentata) ecosystems, in particular, are greatly reduced in distribution as land has been converted to agriculture and other land uses. The fire regime, relative proportions of shrub and grassland patches, and the effects of repeated burns in this ecosystem are poorly understood. We quantified postfire patterns of vegetation accumulation and modeled potential fire behavior on sites that were burned and first measured in the late 1980s at John Day Fossil Beds National Monument, Oregon, USA. The area partially reburned 11 yr after the initial fire, allowing a comparison of one vs. two fires. Repeated burns shifted composition from shrub‐dominated to prolonged native herbaceous dominance. Fifteen years following one fire, the native‐dominated herbaceous component was 44% and live shrubs were 39% of total aboveground biomass. Aboveground biomass of twice‐burned sites (2xB; burned 26 and 15 yr prior) was 71% herbaceous and 12% shrub. Twenty‐six years after fire, total aboveground biomass was 113–209% of preburn levels, suggesting a fire‐return interval of 15–25 yr. Frequency and density of Pseudoroegneria spicata and Festuca idahoensis were not modified by fire history, but Poa secunda was reduced by repeated fire, occurring in 84% of plots burned 26 yr prior, 72% of plots burned 15 yr prior, and 49% in 2xB plots. Nonnative annual Bromus tectorum occurred at a frequency of 74%, but at low density with no differences due to fire history. Altered vegetation structure modified fire behavior, with modeled rates of fire spread in 2xB sites double that of once‐burned sites. This suggests that these systems likely were historically composed of a mosaic of shrub and grassland. However, contemporary increases in fire frequency will likely create positive feedbacks of more intense fire behavior and prolonged periods of early‐successional vegetation in basin big sagebrush communities.